Displays and lamps using LEDs for illumination are becoming increasingly popular in commercial and residential environments. LEDs provide a number of advantages over traditional light sources, such as fluorescent bulbs, which are common for most applications such as low power consumption, long lifetime, and no hazardous material, and additional specific advantages for different applications. For instance, LEDs are rapidly replacing Cold Cathode Fluorescent Lamps (CCFL) as LCD backlights due to smaller form factor and wider color gamut. LEDs for general illumination provide the opportunity to adjust the color or white color temperature for different effects. LED billboards are replacing paper billboards to enable multiple advertisements to timeshare a single billboard.
LEDs are popular for such display and lamp applications due to the low cost, high energy efficiency, and long lifetime, however, variations in light output between individual LEDs and between LEDs from each color component group can limit performance and increase cost. For instance, the amount of light produced by an LED for a given current can vary by a factor of two to one or more between LEDs within a manufacturing lot and between lots, which when combined with the light produced by different color LEDs in a display pixel or LCD backlight for instance, the blended color produced can vary tremendously. Likewise, the wavelength of the light produced by such LEDs can vary by 20 nm or more which produces a clearly visible color shift. Consequently, LED vendors typically sort LEDs into groups or bins with narrower specifications. LED customers may either purchase only specific bins for a higher price or design products that can tolerate wider tolerances, which may limit performance.
Further, an LED array that is designed and calibrated to produce uniform brightness and color when manufactured will degrade with use. As LEDs age, the light intensity produced for a fixed current may increase or decrease over some amount of time and then will continue decreasing until end of life. Different color LEDs have different average aging characteristics, which may varying widely between individual LEDs. Consequently, perfectly built LED arrays will develop a grainy appearance with a different hue over time.
Displays with such arrays typically are recalibrated periodically over time to compensate for changing LED characteristics, which can be costly and time consuming. For instance, stadium displays are apparently re-calibrated every two years, which takes a team of people days to complete. Special cameras with telescopes focus on each individual LED to determine brightness and color, which is fed back to the display controller. Billboards, which operate continuously, should be re-calibrated more frequently to maintain optimum performance, which is costly and produces downtime.
LED backlights for LCD televisions, computers, and mobile phones for instance produce white light from either phosphor coated blue LEDs or a combination of multi-color LEDs, such red, green, and blue. Such light typically passes through a diffusing layer before being applied to the back of the liquid crystal layer, which combines the light from the LEDs to produce uniform light behind the liquid crystal. Smaller displays typically have LEDs placed along one side of the display and inject light into specially shaped waveguides behind the diffuser, while larger displays have arrays of LEDs behind the liquid crystal layer that match the physical dimensions of the display and typically use thin diffusing elements to produce uniform light from the LED point sources.
Displays with arrays of LEDs for backlighting have at least two advantages over displays with LEDs along one or more sides. First, illumination generally is more uniform across the display, and second, the illumination from each LED in the array can be independently adjusted to improve the contrast ratio, which is called “local dimming” in the industry. However, illumination is more uniform only if the light output from each LED, or combination of LEDs for RGB backlights for instance, is the same. As in LED billboards and stadium displays, such arrays can be calibrated during manufacturing, but illumination uniformity and color, in particular with RGB backlights, will change over time for the reasons previously discussed.
LCD backlighting from red, green, and blue LEDs provides many advantages over lighting from white LEDs including wider color gamut and potentially substantially lower power consumption and simpler designs. Conventional LCD displays with white backlights use a layer of color filters to produce red, green, and blue light from the three liquid crystal elements in each pixel. Such color filters add cost and complexity to the design of the display and possibly more importantly for mobile devices block most of the light produced by the backlight. Displays using RGB backlighting can eliminate the color filters by sequencing the colors produced using a technique known in the industry as Field Sequential Color (FSC). Such an approach uses one liquid crystal element per pixel instead of the conventional three and is updated three times as fast sequentially with the red, green, and blue pixel data. Each color of the backlight is flashed sequentially to match the pixel data currently applied to the liquid crystal element without any of the light power blocked by the color filters. Consequently, the RGB backlight can produce substantially less light using substantially less battery power for the same display brightness than a white LED backlight.
The problem with RGB backlights is producing and maintaining the desired color and intensity produced by each red, green, and blue triplet. LCDs with RGB backlights along one or more sides typically use photo-sensors to detect the average intensity of each color component, which is fed back to the LED driver circuitry to maintain the proper mix of colors, however, such photo-sensors and associated feedback circuitry is costly.
High end large screen LCD televisions with LED backlighting have recently been introduced by companies such as Samsung and Sony, which have arrays of LEDs that enable local dimming for high contrast ratios. At least some of such Samsung products have arrays of white LEDs, while at least some of such Sony products have arrays of RGB LEDs to support a wider color gamut. It is unclear how Sony maintains the proper color point, but such Sony products are sold for substantially more than such Samsung products. The relatively simple approach described previously for RGB LED backlights along one or more sides of the display that uses one or more sets of photo-sensors to detect the intensity of each color component is not possible with such Sony products that support local dimming. Many sets of photo-sensors distributed throughout the array of RGB LEDs would be necessary to maintain uniform brightness and color across such a display.
LED lamps for general illumination typically use blue LEDs covered with a phosphor that absorbs most of the blue photons and re-emits photons with longer wavelengths to produce white light, which produces energy loss proportional to the wavelength shift. Phosphors that shift significant photons towards the red end of the spectrum (long wavelength) produce a desirable so called “warm light” but are the least energy efficient. Additionally, over time the phosphors can degrade causing the light produced to become bluer over time.
To overcome the inefficiencies of producing warm light comparable to that produced by an incandescent lamp, some LED lamps use a combination of phosphor coated blue LEDs to produce the blue to yellow wavelengths and red LEDs to produce the red. Such lamps generally perform well, but are expensive due to the complex circuitry necessary to produce the proper mix of light from the white LEDs versus the red LEDs. Any shift in the balance produces a shift in white color temperature and is particularly noticeable when viewed next to another lamp with a slightly different color temperature. The circuit that controls the color temperature typically includes photodiodes in a feedback loop to control the currents provided to the different colored LEDs.
Lamps that use combinations of red, green, and blue LEDs to produce light can be more energy efficient and can produce a much wider range of colors than lamps with just phosphor coated blue or phosphor coated blue plus red LEDs, however, producing and maintaining a fixed color point is more difficult. Such lamps require complex optical feedback mechanism that monitor and adjust each color component during manufacturing and over operating lifetime of such a lamp.
A need exists for a means to maintain uniform brightness and color across an array of multi-color LEDs, such as RGB, used to directly produce images in the case of LED billboards and stadium displays, for instance, and indirectly in the case of LCD backlighting, without the expense and complexity of on site pixel by pixel calibration in the case of LED billboards and stadium displays for instance, and special photo-sensors in the case of LCD backlights. Likewise, there is a need to produce and maintain a fixed color or color temperature from an LED lamp. In all cases, a need exists to minimize the need for binned LEDs.